The use of NMR techniques in measurement, detection and imaging has become desirable in many scientific fields of endeavor. The non-invasive, non-destructive nature of NMR has facilitated application to industrial instrumentation, analysis and control tasks, in a variety of applications, including but not limited to cosmetics, perfumes, industrial chemicals, biological samples and food products. As one example, check weighing is used by the pharmaceuticals industry for monitoring and regulating the amount of drug in a sealed glass vial during filling. The drug weight can be as small as a fraction of a gram, and is required to be weighed with an accuracy of a few percent or better, in a vial weighing tens of grams at a rate of several weighings per second.
International Patent Application No. WO 99/67606, incorporated herein by reference as if fully written out below, describes a check weighing system for samples on a production line using NMR techniques. This system includes a magnet for creating a static magnetic field over an interrogation zone to produce a net magnetisation within a sample located within the interrogation zone, and a RF coil for applying an alternating magnetic field over the interrogation zone to cause excitation of the sample according to the principles of NMR.
As is well known in the NMR art, after pulse excitation of the sample by the alternating magnetic field, the sample emits a signal induced in the RF coil, called the free induction decay (FID), from which much information, like sample mass (or weight) can be learned. The FID is directly proportional to the net magnetisation applied to the sample. However, if more than one sample is located on the product filling line within the interrogation zone, and particularly within the alternating magnetic field when a sample is being excited, the additional sample or samples will also develop their own net magnetisation, and emit their own FIDs induced in the RF coil. Consequently, the FID induced in the RF coil in this circumstance is an aggregate of FIDs in which the neighboring samples produce an interference or cross coupling effect. An accurate determination of the mass (or weight) of the single test sample requires that cross coupling effects be minimized or eliminated.
Of course, spacing samples on the production line so that only one sample is located within the interrogation zone at a time would eliminate the potential for cross coupling. This is undesirable from a manufacturing standpoint because it would reduce the rate at which samples may be processed.
It is desirable to provide a method for reducing or eliminating the effects of near neighbor samples on the NMR measurement of the mass of a sample in a NMR check weighing system for samples on a production line.